Abstract

We report a study on the mechanical properties of magnetorheological (MR) fluids prepared with porous iron particles with rough surfaces. These particles were obtained by reducing a magnetite precursor in a H2 atmosphere at 400 °C. Small-amplitude dynamic oscillatory and steady shear flow measurements were carried out in the presence of external magnetic fields. Results were compared with those obtained for MR fluids prepared with conventional solid carbonyl iron particles of comparable size. We found significant differences between the rheology of both types of suspensions, and, more importantly, we found that simple available models can predict quantitatively those differences as long as the average density of the particles is known and is used to calculate their effective volume magnetization and the real volume fraction of the MR fluids prepared with them. By doing so, we obtained for both the porous ironsuspensions and the solid ironsuspensions a single master curve of the dimensionless storage modulus at saturation as a function of volume fraction (φ), and a good collapse of the viscosity versus Mason number curves as well as of the yield stress versus applied field curves. Particle porosity is thus an important factor in MR fluids and should be considered in their design and modeling. Finally, the porous ironsuspensions also exhibited an atypical thickening behavior that was not observed in the solid iron analogues and that we tentatively ascribe to the rougher surface of the porous particles.

Received 08 March 2011Accepted 02 August 2011Published online 23 September 2011

Acknowledgments:

Fernando Vereda is especially grateful to the “Programa de reincorporación de doctores de la U. de Granada.” The authors would like to thank M. P. Morales for carrying out the chemical reduction of the magnetite particles, and Prof. F. Galisteo-González for providing the Bool2k software used for the generation of particle-size distributions from electron microscopy micrographs. This work was supported by MICINN MAT 2010-15101 project (Spain), by the European Regional Development Fund (ERDF) and by Junta de Andalucía P10-FQM-5977 and P10-RNM-6630 projects (Spain). J. P. Segovia-Gutiérrez acknowledges financial support by the “Ministerio de Educación: Becas del Programa de Formación del Profesorado Universitario (FPU)” (AP2008-02138).